1. Field of the Invention
The present invention relates to a virtual image optical system suitable for use in view finders of video cameras, head mount displays or the like. In particular, the present invention relates to a virtual image optical system using a reflective spatial light modulator.
2. Description of the Related Art
The spatial light modulator (SLM) is a device which is applied to be supplied with a video signal and to modulate light on every pixel on the basis of image data of the video signal.
The spatial light modulators can be classified into a transmission type which modulates light transmitted by the spatial light modulator and a reflection type which modulates light reflected by the spatial light modulator. The virtual image optical system of the present invention uses the latter reflective spatial light modulator.
The spatial light modulators use liquid crystal, digital micro mirrors, or the like. Especially, spatial light modulators using liquid crystal are referred to as liquid crystal spatial light modulators.
The liquid crystal can be classified to twisted nematic mode type, birefringence mode type, and light dispersion mode type, and light absorption mode type.
As typically used liquid crystal, there are TN liquid crystal using the twisted nematic (TN) operation mode of the twisted nematic mode type, STN liquid crystal using the super twisted nematic (STN) operation mode of the birefringence operation mode type, and FLC liquid crystal using the ferroelectric liquid crystal (FLC) operation mode.
By referring to FIG. 1, the structure and operation principle of a reflective spatial light modulator using the TN liquid crystal or the STN liquid crystal will now be described.
A TN or STN liquid crystal reflective spatial light modulator 90 includes a pair of electrode portion, a liquid crystal material 95 inserted between the electrode portion, and a lower reflector 96.
The upper electrode portion includes a glass substrate 91A, a transparent electrode 92A disposed inside (under) the glass substrate 91A, and an alignment layer 93A disposed inside (under) the transparent electrode 92A. The lower electrode portion includes a glass substrate 91B, a transparent electrode 92B disposed inside (on) the glass substrate 91B, and an alignment layer 93B disposed inside (on) the transparent electrode 92B.
A polarizer 94A is disposed outside (on) the glass substrate 91A of the upper electrode portion. An analyzer 94B is disposed outside (under) the glass substrate 91B of the lower electrode portion.
Directions of the polarization of the two polarizers 94A and 94B are perpendicular to each other.
Each of the alignment layers 93A and 93B has a function of aligning the alignment direction of molecules of the liquid crystal material 95. The alignment direction of the alignment layer 93A disposed in the upper electrode portion is parallel to the polarization direction of the polarizer 94A disposed in the upper electrode section. The alignment direction of the alignment layer 93B disposed in the lower electrode portion is parallel to the polarization direction of the analyzer 94B disposed in the lower electrode portion.
In other words, the alignment directions of the two alignment layers 93A and 93B are perpendicular to each other.
FIG. 1A shows a voltage non-application state in which a voltage is not applied to each of the transparent electrodes 92A and 92B. FIG. 1B shows a voltage application state in which a voltage is applied to each of the transparent electrodes 92A and 92B.
In the voltage non-application state of FIG. 1A, the alignment of molecules of the liquid crystal material 95 comes in a twisted state. In the voltage application state of FIG. 1B, molecules of the liquid crystal material 95 come in a aligned state in the vertical direction.
In the case of TN liquid crystal, the twist angle of molecules in the voltage non-application state is 90 degrees.
In the voltage non-application state of FIG. 1A, polarized light 97A fed from the upper polarizer 94A is rotated in direction of the polarization by passing through the liquid crystal material 95.
Therefore, this polarized light 97B is passed through the lower polarizer 94B and arrives at the reflector 96.
In the same way, polarized light 97C reflected by the reflector 96 is rotated in direction of the polarization by passing through the liquid crystal material 95, and the polarized light 97C is passed through the upper polarizer 94A.
In other words, the polarized light 97C returns to the same path as the incident light.
In the voltage application state of FIG. 1B, the polarized light 97A fed from the upper polarizer 94A is not rotated in direction of the polarization by passing through the liquid crystal material 95.
Therefore, this polarized light cannot be passed through the lower polarizer 94B, and does not arrive at the reflector 96.
In other words, reflected light for the incident light is not obtained.
By referring to FIGS. 2A to 2C, the structure and operation principle of a reflective spatial light modulator using FLC will now be described.
An FLC reflective spatial light modulator 100 includes a pair of electrode portions, and a liquid crystal material 105 inserted between the electrode portions.
The upper electrode portion includes a glass substrate 101A, a transparent electrode 102A disposed inside (under) the glass substrate 101A, and an alignment layer 103A disposed inside (under) the transparent electrode 102A. The lower electrode portion includes a silicon substrate 101B, an aluminum electrode 102B disposed inside (on) the silicon substrate 101B, and an alignment layer 103B disposed inside (on) the aluminum electrode 102B.
The aluminum electrode 102B functions as a reflective layer as well.
A polarizer 104 is disposed outside (on) the glass substrate 101A of the upper electrode portion.
FIG. 2A shows a first voltage direction state in which a voltage in a first direction is applied to each of the transparent electrode 102A and the aluminum electrode 102B. FIG. 2B shows a second voltage direction state in which a voltage in a second direction is applied to each of the transparent electrode 102A and the aluminum electrode 102B.
As shown in FIG. 2C, the liquid crystal material 105 does not exhibit a birefringence effect to the incident polarized light in the first voltage direction state, but the liquid crystal material 105 exhibits a birefringence effect to the incident polarized light in the second voltage direction state.
In the first voltage direction state of FIG. 2A, the liquid crystal material 105 does not exhibit a birefringence effect, and consequently the polarized light 107A fed from the polarizer 104 is passed through the liquid crystal material 105 and arrives at the aluminum electrode (reflective layer) 102B without changing the state of polarization.
The polarized light 107B reflected by the aluminum electrode (reflective layer) 102B is passed though the liquid crystal material 105 again and arrives at the polarizer 104 without changing the state of polarization.
In other words, the light having the same polarization state as that of the incident light returns to the polarizer 104.
As a result, exit light is obtained from the polarizer 104.
On the other hand, in the second voltage direction state of FIG. 2B, the polarized light 107A fed from the polarizer 104 is subjected to a birefringence effect when it is passed through the liquid crystal material 105, and consequently linearly polarized light is changed to circularly polarized light.
The circularly polarized light is reflected by the aluminum electrode (reflective layer) 102B and thus the rotation direction of the circularly polarized light 107B becomes reverse.
The circularly polarized light 107B with reverse rotation direction is subjected to a birefringence effect when it is passed through the liquid crystal material 105 again, and consequently the circularly polarized light 107B is changed to a linearly polarized light.
This linearly polarized light is perpendicular to the polarization direction of the polarizer 104 and therefore is not passed through the polarizer 104.
FIG. 3 shows an example of a virtual image optical system having a reflective spatial light modulator which uses FLC (ferroelectric liquid crystal).
This virtual image optical system includes an FLC reflective spatial light modulator 100, a polarizer 104, a half mirror 111, and an analyzer 112.
The polarization direction of the polarizer 104 and the polarization direction of the analyzer 112 are perpendicular to each other.
The FLC reflective spatial light modulator 100 is the same as the FLC reflective spatial light modulator shown in FIGS. 2A and 2B, and its detailed description will be omitted.
The spatial light modulator is typically used together with an illuminating light source device.
Light 107A fed from an illuminating light source device which is not illustrated arrives at the half mirror 111 via the polarizer 104.
The light 107A reflected by the half mirror 111 arrives at the half mirror 111 again via the FLC reflective spatial light modulator 100.
Polarized light 107B passed through the half mirror 111 arrives at the analyzer 112.
In the first voltage direction state, the liquid crystal material 105 does not exhibit a birefringence effect as described above. Therefore, the polarized light 107A fed from the polarizer 104 arrives at the analyzer 112 without changing the state of polarization.
Therefore, light fed from the illuminating light source device is not passed through the analyzer 112.
In the second voltage direction state, however, the liquid crystal material 105 exhibits a birefringence effect. Polarized light 107B fed from the polarizer 104 is changed in polarization state, and arrives at the analyzer 112.
Therefore, light fed from the illuminating light source device is passed through the analyzer 104 and is outputted.
The reason why an illuminating light source device is used is that normally black should be formed when light from the illuminating light source device is not subjected to the birefringence effect by the liquid crystal material 105.
This is because the phase difference generated by the birefringence effect of the liquid crystal material 105 depends upon the layer thickness of the liquid crystal material 105 and the incidence angle of the incident light.
By referring to FIG. 4, an example of a conventional virtual image optical system using a reflective spatial light modulator will now be described.
This example is described in U.S. Pat. No. 5596451. For details, see the U.S. Patent.
The virtual image optical system of the present example has a polarization beam splitter cube 125 (hereafter simply referred to as a cube). A polarization beam splitter 125E is formed on a diagonal plane of the cube 125.
As illustrated, an illuminating light source device 121 and a polarizer 123 are disposed so as to correspond to a first plane 125A of the cube 125. A reflective spatial light modulator 122 is disposed so as to correspond to a second plane 125B. A quarter wave/undulation plate 126 and a reflective mirror 127 are disposed so as to correspond to a third plane 125C.
Light fed from the illuminating light source device 121 is passed through the polarizer 123, and deflected by the polarization beam splitter 125E. The light fed from the illuminating light source device 121 thus arrives at the reflective spatial light modulator 122.
Modulated reflected light is outputted from the reflective spatial light modulator 122.
This reflected light is passed through the polarization beam splitter 125E and the quarter wave/undulation plate 126, and reflected by a concave reflective surface of the reflective mirror 127.
This reflected light is passed through the quarter wave/undulation plate 126 again, and deflected by the polarization beam splitter 125E as represented by 128A. Thus the reflected light arrives at a human pupil 131 in a viewing area 130.
The virtual image optical system of the present example has the following drawbacks.
(1) As represented by a broken line 128B, a part of light fed from the illuminating light source device 121 directly arrives at the pupil 131 as stray light.
This stray light 128B becomes noise of image data displayed by the reflective spatial light modulator 122, and lowers the contrast of the image information.
(2) The polarization beam splitter cube 125 of a cubic shape is used. Therefore, the incidence plane 125A of light fed from the illuminating light source device 121 is planar. The plane 125B corresponding to the reflective spatial light modulator 122, and the plane 125C corresponding to the quarter wave/undulation plate 126 and the reflective mirror 127 are also planar. A plane 125D through which light exits toward the viewing area 130, and the polarization beam splitter plane 125E are also planar. There are few planes having a degree of optical freedom.
For example, in the case where the pixel pitch of the reflective spatial light modulator 122 is small, therefore, a sufficient resolution cannot be obtained.
(3) In the case where it is assumed that the dimensions of the reflective spatial light modulator 122 are fixed, the angle of view must be made large for making the dimensions of the apparent display screen large.
If the angle of view is made large, the focal length f becomes small.
If the focal length f is made smaller, the absolute value of the Petzval sum PS becomes large.
In other words, the field curvature becomes large.
The Petzval sum PS is a parameter representing the field flatness or field curvature of the image. As represented by the following equation, the Petzval sum PS is a function of the refractive index n and the focal length f. EQU PS=.SIGMA.(1/nf) (1)
If the Petzval sum is zero, i.e., PS=0, then it is meant that the image is flat.
The larger the absolute value of the the Petzval sum is, the larger value the field curvature of the image has.
In the example of FIG. 4, only the concave reflective surface of the reflective mirror 127 bears the refractive power of the whole optical system, and n=-1 and f&gt;0.
Therefore, the Petzval sum becomes PS&lt;0.
(4) Since the polarization beam splitter cube and the reflective mirror are separate components, the number of components increases and the manufacturing cost becomes high.
Furthermore, since there is an air layer between the cube and the reflective mirror, the focal length becomes long as compared with the case where there is no air layer.
(5) Since the polarization beam splitter cube is high in manufacturing cost and heavy in weight, the manufacturing cost and the weight of the system increase.
By referring to FIG. 5, another example of conventional virtual image optical system using a reflective spatial light modulator will now be described.
This example is also described in U.S. Pat. No. 5,596,451. For details, see the U.S. Patent.
The virtual image optical system of the present example has such a configuration that a cube-shaped polarization beam splitter cube 124 (hereafter simply referred to as a cube) having small dimensions is additionally provided in the virtual image optical system described with reference to FIG. 4.
The cube 124 having small dimensions has a configuration similar to that of the cube 125 having large dimensions. A polarization beam splitter 124E is formed on a diagonal plane of the cube 124.
As illustrated, an illuminating light source device 121 is disposed so as to correspond to a first plane 124A of the cube 124 having small dimensions. A reflective spatial light modulator 122 is disposed so as to correspond to a second plane 124B. The cube 125 having large dimensions is disposed so as to correspond to a third plane 124C. A quarter wave/undulation plate 126 and a reflective mirror 127 are disposed so as to correspond to a third plane 125C of the cube 125 having large dimensions.
Light fed from the illuminating light source device 121 is deflected by the polarization beam splitter 124E of the small cube 124. The light fed from the illuminating light source device 121 thus arrives at the reflective spatial light modulator 122.
Modulated reflected light is outputted from the reflective spatial light modulator 122.
This reflected light is passed through the two polarization beam splitters 124E and 125E and the quarter wave/undulation plate 126, and reflected by a concave reflective surface of the reflective mirror 127.
This reflected light is passed through the quarter wave/undulation plate 126 again, and deflected by the polarization beam splitter 125E as represented by 128A. Thus the reflected light arrives at a human pupil 131 in a viewing area 130.
In the virtual image optical system of the present example, a part of light fed from the illuminating light source device 121 is passed through the polarization beam splitter 124E as represented by a broken line 128B, but it does not arrive at the viewing area 130.
Therefore, noise is not generated in the image data by stray light.
In the present example, therefore, the first drawback (1) among the above-described three drawbacks is avoided. However, other drawbacks still remain.
By referring to FIGS. 6A and 6B, another example of conventional virtual image optical system will now be described.
The virtual image optical system shown in FIG. 6A includes an illuminating light source device 141, an FLC reflective spatial light source device 142A, a polarization beam splitter 143, and a concave reflective mirror 144.
The polarization beam splitter 143 of the present example has such a structure that a polarization beam splitter layer is applied onto a glass substrate.
In the virtual image optical system shown in FIG. 6B, a TN liquid crystal reflective spatial light modulator 142B is used as the spatial light modulator, and further a quarter wave/undulation plate 145 is disposed between the polarization beam splitter 143 and the concave reflective mirror 144.
Light fed from the illuminating light source device 141 arrives at the reflective spatial light modulators 142A or 142B via the polarization beam splitter 143.
The light fed from the reflective spatial light modulators 142A or 142B is deflected by the polarization beam splitter 143, and reflected by the reflective mirror 144. The light thus arrives at a human pupil 131 in a viewing area 130.
In the example of FIGS. 6A and 6B, a platelike polarization beam splitter 143 is used instead of the cube-shaped polarization beam splitter cube.
Therefore, the above-described fifth drawback is avoided.
However, this example has the following drawbacks.
(1) In the case where dimensions of the optical systems are the same, the focal length of the reflective mirror which generates the refractive power becomes longer and the optical magnification becomes smaller as compared with the case where the cube is used.
If the optical magnification becomes smaller, dimensions of the apparent displayed image become smaller.
In the case where the virtual image distance is infinite, the focal length for focusing a virtual image is a value obtained by dividing a physical distance between the reflective spatial light modulator and the principal point of the reflective mirror by the refractive index of a medium between them.
(2) In the platelike polarization beam splitter, it is difficult to make both the P-and S-polarization transmission factors high in the whole visible light band.
Even if the reflective TN liquid crystal spatial light modulator is used as the reflective spatial light modulator and the quarter wave/undulation plate is disposed between the reflective mirror and the polarization beam splitter as in the example shown in FIG. 6B, the light utilization efficiency cannot be made sufficiently high.
(3) For example, as compared with the polarization beam splitter of internal filling type in which the configuration surfaces can be made curved surfaces, there are fewer surfaces having a degree of optical freedom.
In some cases, therefore, resolution and aberration correction such as field curvature cannot be achieved sufficiently.
By referring to FIG. 7, another example of conventional virtual image optical system will now be described.
The virtual image optical system of the present example includes an illuminating light source device 10, an optical film 15, a reflective spatial light modulator 20, first and second half mirrors 21 and 22, and a concave reflective mirror 23.
The virtual image optical system further includes a control circuit. The control circuit includes a system controller 2 supplied with a video signal 1, an illuminating light source device drive circuit 4 supplied with a signal fed from the system controller 2, and a reflective spatial light modulator drive circuit 6 supplied with a signal fed from the system controller 2.
On the basis of a signal fed from the system controller 2, the illuminating light source device drive circuit 4 supplies a light source drive signal to the illuminating light source device 10. On the basis of a signal fed from the system controller 2, the reflective spatial light modulator drive circuit 6 supplies a drive signal to the reflective spatial light modulator 20.
The illuminating light source device 10 includes a light source 11, a reflector 12, a light pipe 13, and a reflector 14.
Light fed from the light source 11 is led into the light pipe 13 directly or by being reflected by the reflector 12.
A part of the light led into the light pipe 13 directly exits through a front surface 13A. However, the remaining part is reflected by two surfaces 13A, 13B, and the reflector 14 repetitively and thereafter exists through the front surface 13A.
By making the section of the light pipe 13 wedge-shaped, the intensity of light which exits from the front surface 13A of the light pipe 13 is made uniform from the area near the light source 11 to the area located far therefrom.
The light from the light pipe 13 arrives at the optical film 15.
The optical film 15 has a function of controlling the divergence angle DA of a beam from the light pipe 13. Its performance is represented by a half divergence angle HDA.
The half divergence angle HDA is an angle equivalent to half of a solid angle at which the light intensity becomes half of its peak value.
Light passed through the optical film 15 is reflected by the first half mirror 21 to arrive at the reflective spatial light modulator 20 and modulated by the reflective spatial light modulator 20.
Light fed from the reflective spatial light modulator 20 is passed through the first half mirror 21, reflected by the second half mirror 22, reflected by the concave reflective mirror 23, passed through the second half mirror 22, and arrives at a pupil 131 in a viewing area 130.
FIG. 8 is a diagram schematically showing the optical system of the virtual image optical system of FIG. 7.
By referring to FIG. 8, the optical system of the virtual image optical system of FIG. 7 will now be described in detail.
As evident from FIG. 8, the optical system of the virtual image optical system basically includes a light source optical system and eyepiece optics.
The light source optical system includes a beam splitter, i.e., a first half mirror 21, for leading light fed from an illuminating light source device 10 into a reflective spatial light modulator 20.
The eyepiece optics includes a beam splitter, i.e., a second half mirror 22, for deflecting light fed from the reflective spatial light modulator 20, and a reflector, i.e., a concave reflector 23.
An angle formed by the reflective spatial light modulator 20 and the beam splitter of the light source optical system, i.e., the first half mirror 21 is denoted by .alpha..
A point of intersection of a ray passing through a center of the reflective spatial light modulator 20, i.e., a chief ray and the beam splitter of the eyepiece optics, i.e., the second half mirror 22 is denoted by P. Furthermore, a point of intersection of the chief ray and the reflector, i.e., the concave reflective mirror 23 is denoted by Q.
A plane normal vector at the point P is denoted by A. A plane normal vector at the point Q is denoted by B. An angle formed by the two vectors A and B is denoted by .beta..
In the present example, .alpha. is 45 degrees and .beta. is 135 degrees.
Furthermore, as illustrated, an angle formed by the vector B and the beam splitter 22 of the eyepiece optics is 45 degrees.
A distance between the point Q on the reflector 23 of the eyepiece optics and the point P on the beam splitter 22 is denoted by f1'. A distance between the reflective spatial light modulator 20 and the point P on the beam splitter 22 is denoted by f2'.
A focal length f' of the reflector, i.e., the concave reflector 23 is equal to the sum of these two distances. EQU f'=f1'+f2' (2)
A distance between the point Q on the reflector 23 of the eyepiece optics and a bottom end of the beam splitter, i.e., the second half mirror 22 is denoted by L'. A distance between the bottom end of the second half mirror 22 and a pupil O in the viewing area 130 is denoted by R'.
The distance L' represents the thickness of the optical system. The distance R' represents a design distance between an eye and the optical system, i.e., an eye relief.
This example has a drawback that the focal distance f' is comparatively long.
Furthermore, the above-described angle .beta. between the two vectors A and B is comparatively small. It is, for example, 135 degrees.
As a result, the thickness distance L' of the optical system cannot be made small.
Furthermore, the eye relief or the distance R' cannot be made large.